The invention relates generally to cooling passages within airfoils and, more particularly, to rejuvenating cooling passages within airfoils of gas turbine blades and gas turbine vanes (or turbine airfoils) to enhance convective cooling thereof. As used herein, the term xe2x80x9cbladexe2x80x9d encompasses both blades and buckets, which two terms are typically used for aircraft engine and land-based applications, respectively. The term xe2x80x9cvane,xe2x80x9d as used herein, means the airfoil portion of a nozzle and encompasses both aircraft engine and land-based applications.
In gas turbine engines, hot gases from a combustor are used to drive a turbine subjecting many components, such as stationary vanes in high pressure turbine nozzles and turbine blades, to high temperatures and stress. The capacity of the engine is limited to a large extent by the ability of the turbine vanes and turbine blades to withstand the resulting temperature and stress.
Typical turbine vanes and blades include an airfoil, over which the combustion gases flow. A vane airfoil is positioned between an outer and an inner band to form the turbine nozzle. In order to decrease vane and blade temperature, thereby improving thermal capability, it is known to supply cooling air to hollow cavities within the turbine airfoils. Typically one or more cooling passages are formed within a turbine airfoil with a coolant (such as compressor discharge air) supplied through an opening in the airfoil and allowed to exit through cooling holes strategically located on an outer surface of the airfoil. The cooling passages provide convective cooling inside the airfoil and film-type cooling on the surface thereof. Many different cavity geometries have been employed to improve heat transfer to the cooling air inside the airfoil. For example, cooling passages typically have circular, racetrack, rectangular, square or oblong transverse cross-sectional shapes.
One known turbine blade airfoil cooling circuit includes a number of unconnected longitudinally-oriented passages (hereinafter xe2x80x9cradial cooling passagesxe2x80x9d) extending for example through an airfoil of a turbine rotor blade. Each radial cooling passage receives cooling air from near a root of the airfoil and channels the air longitudinally toward a tip of the airfoil. Other cooling circuits are serpentine, comprising a number of longitudinally-oriented passages which are series-connected to produce serpentine flow. For either cooling circuit, some air exits the airfoil through film cooling holes near the airfoil""s leading edge and some air exits the airfoil through trailing edge cooling holes.
Turbine vanes narrow in thickness to a relatively narrow trailing edge. Consequently, cooling the trailing edge is difficult. To cool the turbine vane, vane airfoils generally include one or more central passages and a row of discharge holes formed in the trailing edge of the turbine vane airfoil. Discharge holes may also be provided in a leading edge of the vane airfoil. Coolant flows into the central passage(s) from the tip and/or root of the vane airfoil and out of the discharge holes. Further, one or more rows of film cooling holes may be provided along a pressure sidewall of the vane airfoil. In addition, a vane airfoil suction sidewall may include several rows of film cooling holes between a leading edge of the vane airfoil and a maximum thickness region thereof.
Modern turbine airfoils often include turbulence promoters (xe2x80x9cturbulatorsxe2x80x9d) and other cooling improvements to enhance heat transfer. However, in the 1960""s and 1970""s, turbine cooling technology in turbine airfoils used in power generation turbines typically involved using shaped tube electrochemical machining (STEM) to drill circular or oval cooling passages in the turbine airfoils. The surfaces of these older STEM drilled cooling passages are typically smooth, without any turbulators.
Numerous turbine airfoils incorporating the older STEM drilled cooling passages remain in service today. These turbine airfoils are often repaired during regularly scheduled maintenance overhauls of power generation turbines. Such maintenance overhauls occur after a period of field service, for example every ten thousand (10,000) service hours. Upon overhaul, generally a number of the turbine airfoils exhibit significant deterioration so as to require repair to support continuing service for the turbine airfoils. Currently turbine airfoil repairs include surface cleaning, coating stripping, crack inspection, crack repair, tip repair, and recoating. These repair processes are performed to restore the airfoil to its original condition to prevent its service life from being cut short due to wear. However, current repair processes do not improve the cooling passages within the turbine airfoils and hence do not enhance the heat transfer of the cooling passages to the coolant. Consequently, the repaired turbine airfoils do not have extended services lives under the original operating conditions, nor do they allow elevated operating temperatures or reduced cooling flow to improve the efficiency of the overhauled turbine engines.
Accordingly, there is a need in the art for a method to rejuvenate cooling passages within turbine airfoils as part of the repair process during the engine maintenance overhaul. Advantageously, rejuvenation of the cooling passages would enhance the heat transfer coefficient of the turbine airfoils. Improved heat transfer provides two related benefits: life enhancement for the turbine airfoil and increased turbine engine efficiency. More specifically, improved heat transfer provides either a cooler turbine airfoil (for the same coolant flow), yielding a longer service life for the airfoil, or alternatively facilitates reduced cooling flow (i.e., bleeding off less compressor air), increasing turbine engine efficiency. There is a corresponding need for turbine airfoils having the rejuvenated radial cooling-passages and for a tool to efficiently rejuvenate the cooling passages.
Briefly, in accordance with an embodiment of the present invention, an electrode for rejuvenating a cooling passage within an airfoil is disclosed. The electrode includes a tip, an end, a conductive core extending between the tip and the end, and an insulating coating disposed on the conductive core. The insulating coating exposes a number of conductive strips of the conductive core extending between the tip and the end. The insulating coating forms a number of insulating portions and further exposes a number of spacer portions of the conductive core longitudinally positioned between the insulating portions. The insulating portions substantially span a distance between the tip and the end and are positioned between the conductive strips.
In accordance with another embodiment, an electrochemical machining method for rejuvenating at least one cooling passage within an airfoil is disclosed. An inner surface of the cooling passage is prepared for electrochemical machining, including removing residue from the inner surface. An electrode is positioned in the cooling passage. The electrode includes a conductive core and an insulating coating, and the insulating coating exposes a number of exposed portions of the conductive core. A groove pattern is machined on the inner surface of the cooling passage using the exposed portions of the conductive core by passing an electric current between the electrode and the airfoil while circulating an electrolyte solution through the cooling passage. The machining produces a rejuvenated cooling passage.
In accordance with an airfoil embodiment, an airfoil includes a tip, a root, a body extending between the tip and the root, and at least one cooling passage formed in the body. The cooling passage has an inner surface and a groove pattern formed on the inner surface and is configured to receive coolant. The groove pattern includes a number of grooves, extending along the length of the cooling passage, a number of fins positioned alternately with the grooves and substantially spanning the length of cooling passage, and a number of connectors. Each connector is longitudinally positioned between two of the fins and connects two of the grooves.